Acrylate pressure-sensitive adhesives are well-known. U.S. Pat. No. Re 24,906 (Ulrich), assigned to the assignee of the present case, discloses alkyl acrylate copolymers which comprise a major amount of alkyl esters of acrylic acid, having from 4 to 14 carbon atoms and which comprise a minor amount (i.e. 3 to 12%, preferably 4 to 8%) of a polar, copolymerizable monomer, such as acrylic acid. Such adhesives are widely popular, as they are readily available, and provide a good balance of tack, shear, and peel properties on a variety of substrates at relatively low cost. However, such adhesives can experience poor solvent resistance and poor adhesion to low energy surfaces, such as polyolefins, silicones, and fluorosilicones.
It is well-known that the use of tackifiers in acrylic PSA formulations enhances the adhesion of the PSAs to low energy surfaces. However, this increase in adhesion occurs at the expense of internal strength.
A need has existed for acrylic adhesives having improved solvent resistance, especially against such fluids as gasoline, jet fuel, hydraulic fluid, and oils. Two main approaches have been tried to achieve this goal. One approach involved the modification of the acrylic polymer backbone. The other approach involved the combination of two elastomers in a hybrid adhesive composition.
EP 0,075,191 (1981), assigned to Daikin Kogyo, describes a solution polymer of pentafluoropropyl acrylates which when coated on polyester yields a tape which experiences less than a 10% adhesion loss when exposed to oil, gasoline, kerosene, and water. This adhesive requires expensive fluorinated acrylate monomers and does not include a second elastomer as part of the composition.
Fluoroelastomers have been incorporated into adhesive compositions in an attempt to yield adhesives having improved solvent resistance. JP 61-314 11 (1986), assigned to Suriibondo, describes a radically polymerizable composition comprising (meth)acrylate or oligomer thereof, fluoroelastomer, and a curing agent. The cured composition which demonstrates improved heat-resistance and oil-resistance can be used as an adhesive and a caulking material. The fluoroelastomers used are vinylidene fluoride/hexafluoropropylene copolymers and the adhesives are ultraviolet light (UV) or anaerobically cured. No disclosure is made of fluoroalkyl siloxane elastomers. JP 63-117085 (1988), assigned to Central Glass, discloses a tacky adhesive comprising a copolymer of vinylidene fluoride and hexafluoroacetone combined with a copolymer of 2-ethylhexyl acrylate and vinyl acetate. The adhesive is purported to be weather and oil resistant. Again, no fluoroalkyl siloxane elastomer was included in the composition. EP 0,390,207 (1990), assigned to Daikin Industries, discloses a vibration damping material obtained by dissolving an amorphous fluorine-containing polymer into acrylic monomer and polymerizing the monomer with or without permitting the resulting polymer to form an interpenetrating polymer network. The composition is reported to provide excellent damping, solvent resistance, oil resistance, and weathering resistance, however, no pressure sensitive adhesive properties are taught.
A need thus exists for an inexpensive pressure sensitive adhesive that is resistant to solvents and adheres well to low energy surfaces, with a balance of tack, adhesion and fluid resistance. We have discovered such a pressure-sensitive adhesive.